This invention relates to a solid state process for converting ceramic material having a polycrystalline structure to a single crystal structure by forming a selected surface topography on a body made of polycrystalline material and heating the polycrystalline material body.
The manufacture of polycrystalline alumina (PCA), and its use for high pressure sodium arc discharge lamps (hereinafter "HPS lamps") is known in the art. U.S. Pat. Nos. 3,026,210; 4,150,317 and 4,285,732 to Coble, Laska et al and Charles et al, disclose the production of a high density PCA body having improved visible light transmission using relatively pure alumina powder and small amounts of magnesia. U.S. Pat. No. 4,285,732 further teaches adding zirconia and hafnia to the magnesia-doped alumina to reduce the chances of precipitating a spinel phase and exaggerated or run away grain growth during sintering. PCA bodies useful as arc tubes for HPS lamps have been made according to the processes in these patents having an average grain size ranging from between 15 microns to 100 microns.
Two major drawbacks associated with the use of PCA arc tubes for HPS lamps are that they are light translucent as opposed to light transparent and the sodium in the arc reacts with the alumina at the grain boundaries to form sodium aluminate, which adversely affects the structural integrity of the tube and shortens lamp life. HPS lamps are being designed for ever increasing internal sodium partial pressure within the PCA arc tube to improve the color rendition and provide a whiter emitted light. However, higher internal sodium pressure further shortens lamp life due to increased rate of sodium loss from the arc chamber. Progressive sodium loss results in a corresponding continual rise in the lamp operating voltage, a decrease of both correlated color temperature and color rendering index, and a color shift from white to pink. Also, the sodium which migrates through the arc chamber wall deposits on the inside wall of the evacuated outer lamp envelope causing a brownish stain on the envelope which, in turn, further reduces the light output of the lamp. These problems are substantially reduced with sapphire (i.e., single crystal alumina) arc tubes.
Sapphire arc tubes useful as the arc chamber for HPS lamps have been made by a number of processes, including a modified Czochralski process known as the edge-defined, film-fed growth (EFG) process developed by Tyco Laboratories, Inc. This process uses a seed crystal and a die on the surface of molten alumina in which a hollow tube is continuously pulled out of the melt through the die. This process is expensive and slow. Another process used to produce single crystal alumina (sapphire) is called the floating zone process in which a PCA feed rod is introduced at a predetermined velocity into a heating zone wherein one or more lasers or other concentrated heat source is focused on the rod to melt the alumina in the zone to produce a "melt volume" of molten alumina. A sapphire fiber is continuously drawn from the melt volume at the desired velocity and the feed rod is moved simultaneously, but at a slower rate so that the process is a continuous one. This process is used primarily to produce sapphire fibers and does not readily lend itself to production of single crystal alumina tubing, although its use for such is disclosed in U.S. Pat. No. 3,943,324.
Japanese Patent Publication 62-28118 of H. Yoshida et al. discloses that sapphire can be synthesized via solid state process by inducing a magnesia concentration gradient along the length of a PCA body to ensure grain growth is initiated at a single point on the PCA body during heat treatment. This magnesia gradient can be produced in the PCA body by doping the green body with magnesia in such a way that there is a magnesia gradient in the PCA body, by using a temperature gradient to create the magnesia concentration gradient, or by thinning a section on the green body. Key to the Yoshida process is that the growth of the single crystal initiate from a single location in the polycrystalline body. Further, it is not known if this process was commercialized and the disclosure on its face appears to present potential difficulties in its implementation. In particular, the Yoshida process would necessarily require that sufficient magnesia be added to the alumina to prevent anomalous grain growth during the initial stages of heat treatment of the alumina body. The example used for illustration in the Yoshida patent application, however, suggests the alumina material be doped with approximately 90 ppm of magnesia. It has been shown that at least 300 ppm of magnesia is required to prevent anomalous grain growth in alumina at temperatures of 1850.degree. C. See, e.g., J. G. J. Peelen "Alumina: Sintering and Optical Properties", Ph.D Thesis, Technical University of Eindhovan, Netherlands, May 1977. It is thus likely that heat treatment of alumina doped with a lesser concentration of magnesia would result in the formation of an alumina body composed of multiple coarse crystals rather than a single crystal.
A need exists for producing sapphire from PCA in a facile and relatively inexpensive manner. Solid state conversion of a polycrystalline ceramic article or body to a single crystal, that is, without melting the structure that is being converted, is desirable so that the single crystal has substantially the same size and shape as the polycrystalline article. A solid state conversion process would make it possible to manufacture single crystal articles having nonuniform, asymmetric and complex shapes as well as simple shapes. It would also be a great improvement to the art if such a process were cost effective in greatly reducing both the energy and the time required to effect the formation of a single crystal ceramic structure from a polycrystalline ceramic structure.